1. Field of the Invention
This invention pertains to the general field of interferometry and apparatus for testing surfaces. In particular, it provides a novel approach for testing transparent thin plates and domes by utilizing multimode lasers as the light source for producing interference fringes.
2. Description of the Prior Art
Fizeau interferometers are used in optical laboratories to test the surface geometry of test samples, such as computer-disk and optical-mirror flatness, in comparison to reference surfaces. As illustrated in schematic representation in FIG. 1, a light source 10 (normally a laser operating in the single mode) produces a beam of light 12 that is passed through a microscope objective 14 and a spatial filter 16, such as a pinhole. The light 12 is then collimated by a very-well corrected collimating objective 18 and directed through a reference surface 20 (normally flat) toward a test surface 22 positioned collinearly (with respect to the light beam) and substantially in parallel to the reference surface at some distance within the coherence length of the light source 10. As those skilled in the art readily understand, the light reflected by the test surface 22 interferes with the light reflected at the reference surface 20 and, according to the principle of superposition, bright interference fringes are produced corresponding to all points on the reference surface where the optical path difference (OPD) of the light is equal to a multiple of its wavelength. A beam splitter 24 is placed between the spatial filter 16 and the collimating objective 18 in order to reflect the fringes to the side, so that they may be observed on a screen or directed to a camera 26 through appropriate lenses 28 for display, and/or to other instrumentation for recording and data processing. The interference fringes so produced are typically used to provide a profile of the tested surface.
Most Fizeau interferometers use lasers operating in the single mode because of their very long coherence length (in the order of tens of meters) which permits the placement of the test surface at a practical distance from the reference surface. Because of the opposing positions of the reference and test surfaces, it is important to retain some separation between the two in order to avoid damage. In addition, because of the characteristics of single-mode light, the separation between the reference and test surfaces is not critical to obtain interference fringes so long as within the coherence length, thus facilitating the process of adjusting the position of a sample to produce fringes.
When utilized to test transparent thin-plate samples having parallel surfaces, single-mode laser Fizeau interferometers generate spurious reflections from the opposite surface of the test sample that produce ghost fringes materially affecting the measurements. Referring to FIG. 2, for example, a single-mode light beam .lambda.1 is reflected from the reference surface R and the test surface T of a test sample 30 to produce the desired interference, herein referred to as IF.sub.RT. At the same time, though, the light is also reflected from the back surface S of the sample, thus producing an interference IF.sub.TS between the light reflected from surfaces T and S, and an interference IF.sub.RS from surfaces R and S. This results from the fact that single-mode laser light produces interference fringes at all points within its coherence length. Therefore, the thickness of the transparent thin-plate test sample 30 is immaterial and the resulting fringes will always represent the cumulative interference of the three sets IF.sub.RT, IF.sub.TS and IF.sub.RS. Thus, when testing the front surface T of a parallel-surface glass plate, one always sees a fixed pattern caused by interference between the front and back surfaces.
In order to avoid the effects of the reflections from the back surface S, people have coated it with paints or oils (antireflective coatings), so as to absorb all incident light and eliminate all reflection from it. This solution is obviously cumbersome and time consuming; therefore, while possibly acceptable for laboratory testing, it is not suitable for rapid testing of commercial products, such as glass computer discs.
As described in a recent article, some investigators have proposed a mathematical solution based on processing interference data generated with two single-mode wavelengths. See de Groot et al., "Laser Diodes Map Surface Flatness of Complex Parts," Laser Focus World, 95-98 (February 1994). The cumulative interference IF.sub.RTS produced by the three surfaces R, T and S is measured with a first wavelength .lambda.1 and with a second wavelength .lambda.2, as shown schematically in FIG. 3. Then the sample 30 is flipped over with the back surface S now facing the reference surface R, as seen in FIG. 4, and the cumulative interference IF.sub.RST is again measured with the two wavelengths .lambda.1 and .lambda.2. Thus, four sets of interference data are generated from which the desired interference IF.sub.RT may be extracted by mathematical manipulation. A similar procedure is described by Okada, K. et al., in "Separate Measurements of Surface Shapes and Refractive Index Inhomogeneity of an Optical Element Using Tunable-Source Phase Shifting Interferometry," Applied Optics, 3280-3284 (August 1990). These methods require double handling of the sample and additional processing time, which also materially affect the speed of testing for commercial applications. Another solution is the use of a light source of short coherence length, such as a white light source L, so that the back surface S may be placed beyond the point where it generates interference with the reference surface. To cause interference, the optical path difference between the test and the reference paths must be within the coherence length l.sub.coh of the system. Therefore, in order to satisfy this requirement when using white light, the reference and the test surfaces must be very close to one another (separated by distances in the order of 10 .mu.m). As illustrated in FIG. 5, by positioning the sample 30 so that the front test surface T is within half the coherence length I.sub.coh and the back surface S is outside of it, the latter surface will not cause interference with the reference surface and, by mathematically filtering out the fixed interference between the surfaces T and S (if necessary), accurate measurements can be made of the test surface T.
For good results the separation between the reference and test surfaces must be less than one half of the coherence length. For example, if a 2 nm bandwidth visible light source is used, the coherence length is about 0.20 mm. Thus, the separation between the surfaces R and T must be less than 0.10 mm, which means that the two surfaces are practically in physical contact. At the same time, for a typical glass plate having a 1-mm optical thickness, the separation between the reference surface R and the back surface S would be between 1.00 mm and 1.10 mm, much greater than one half the coherence length of the instrument. Thus, no interference between the reference and back surfaces is obtained.
If a narrower bandwidth is chosen, the coherence length and hence the separation can be greater, but the reflection from the back surface S of the glass plate may interfere with the reference and the test beams. Therefore, short coherence lengths are required for this type of application and the sample must necessarily be placed in very close proximity to (or contact with) the reference surface. This requires careful handling of the sample to avoid scratching of the surfaces, which is cumbersome and time consuming. Therefore, this solution is also not suitable for rapid testing in a production environment.
Accordingly, it would be very desirable to have a simpler and more practical method and apparatus for testing the surfaces of thin transparent plates and domes without interference from the opposite surface of the sample. This invention provides such a method and apparatus utilizing multimode lasers as the interferometer's light source. While multimode lasers have been used in some Michelson interferometry applications for the narrow coherence length they can produce, this property has not heretofore been recognized as the viable and practical solution it can be for the problems outlined above.